1. Field of the Invention
This invention is a new type of evaporative cooler for air-conditioning and refrigeration.
2. The Prior Art
There are three basic kinds of condensers in air-conditioning and refrigeration: air-cooled, water-cooled, and evaporatively cooled.
The most common kind of condenser is air-cooled which residential and smaller commercial equipment use almost exclusively. These condensers typically consist of a coil of copper tubing with refrigerant inside and aluminum fins on the outside of the tubes. A fan blows air over the coil to reject heat to the atmosphere. This type of condenser is simple and requires little maintenance, but is relatively inefficient.
Water-cooled condensers are the second most common type. Water-cooled condensers are typically used with centrifugal and screw chillers found in large commercial and industrial locations. The normal arrangement is a shell-tube heat exchanger with cooling water circulating inside tubes and condensing refrigerant on the outside. The water for this type of condenser is normally cooled by a separate cooling tower. A cooling tower consists of a water spray system that distributes water over a fill material. A fan moves air over the wet fill to cool the water. This system is normally more energy-efficient, with condensing temperatures 20 to 30 degrees Fahrenheit lower than a comparable air-cooled condenser. On the other hand water-cooled condensers and associated cooling towers normally require much more maintenance, are more complicated, and must be assembled in the field rather than shipped as single package.
The third type of condenser is evaporatively cooled. An evaporative condenser combines a water-cooled condenser and a cooling tower into a single package. Existing evaporative condensers are typically found in large commercial or industrial refrigeration systems and are only rarely used in air-conditioning applications.
FIG. 1 shows a typical arrangement for the third type, i.e. an evaporative condenser. A pump 10 draws water from a sump 12 and delivers it to a spray header 15 from which it is sprayed over tubes 14 containing condensing refrigerant. A fan 16 moves air entering at C through the water spray and wet tubes 14 to remove heat therefrom by evaporation. The air exiting the condenser at D goes through a mist eliminator 18 that removes most of the water droplets. While FIG. 1 illustrates a blow through type evaporative condenser, a draw-through type evaporative condenser, wherein the fan is located downstream of the tubes and draws the through the tubes and spray, is also known in the art.
The water pump and spray piping of a system such as shown in FIG. 1 create two major maintenance problems. First they are vulnerable to freezing damage. A partial solution to this problem is to place the sump and pump inside a heated building, but this arrangement makes installation more difficult. The second problem is that the pump and piping can easily clog with dirt. Water strainers are used to reduce this problem, but they too can clog and require frequent maintenance.
Current evaporative condensers and cooling towers have also had significant problems as sources of Legionnaire's disease, a potentially fatal type of pneumonia. Without regular water treatment, the warm, wet conditions in the condenser can support growth of legionella, the bacteria that cause Legionnaire's disease. While legionella are common in freshwater ponds and other surface waters, they do not cause pneumonia unless they are inhaled into the lung. A real problem with conventional evaporative condensers and cooling towers is that the water spray creates a mist of water droplets that can be easily inhaled. Evaporative condensers have been implicated in several outbreaks of Legionnaire's disease.
Evaporative condensers and cooling towers have inherent efficiency advantages compared with air-cooled equipment. For air-cooled equipment the limiting air temperature is the outdoor dry-bulb temperature. For evaporative systems, on the other hand, the limit is the wet-bulb temperature which can be 20 to 40 F. cooler. In addition, heat transfer between air and a wet surface is several times higher than that for a dry surface. The air flow requirements are also smaller with evaporative heat exchange, since the water vapor greatly increases the enthalpy (energy content) of the air. These factors mean that an evaporative condenser or cooling tower can give much lower condensing temperatures while reducing size and fan-energy requirements compared with air-cooled systems.
Despite the better efficiency of evaporative heat exchange, there has been a gradual movement away from water-cooled condensers and evaporative condensers. Early residential air-conditioning systems in the 1930's were normally water-cooled, whereas since the 1950's they have been almost exclusively air-cooled. Cooling loads of 150 tons were normally handled with a water-cooled chiller in the 1960's are now usually serviced with air-cooled equipment. These changes are driven by concerns about maintenance costs associated with water-cooled equipment.
Theoretical analysis and simulation of evaporative condensers, cooling towers, and fluid coolers have been performed by Webb and Villacres, "Performance Simulation of Evaporative Heat Exchangers--(Cooling Towers, Fluid Coolers and Condensers)." AIChE Heat Transfer Symposium, vol. 80, 1984. The theoretical basis is well-established, and their simulation predicted heat load within .+-.3% for a wide variety of air inlet conditions. Moreover, several papers exist which attempt to determine the energy saving potential of evaporatively-cooled condensers. Guinn and Novell "Operating Performance of A Water Spray on an Air-Type Condensing Unit," ASHRAE Transactions, vol. 87, part 2, 1981, reported tests on a commercially available water spraying device on the air-cooled condensing unit of a three ton split system air conditioner. They found that the compressor power input decreased by 5% to 9%, the cooling capacity increased by 4.4% to 8.8%, and that the energy efficiency ratio (EER) of the system was improved by 12% to 19% depending upon the thermodynamic state of the inlet air. The sprayer used 51.5 liters of water per hour. Problems encountered were water runoff from the tubes and fouling and corrosion of the tubes. Markoski, M. J. "Exergetic Analysis of Water Spray Augmentation of Air Cooled Condensers," Proceedings of 19th Intl. Congress of Refrigeration, IIIa, 1995, provides a brief exergetic analysis of this method of heat transfer augmentation. Leidenfrost, W., and B. Korenic, "Evaporative Cooling and Heat Transfer Augmentation Related to Reduced Condenser Temperature", Heat Transfer Engineenng, vol. 3, 1982, tested evaporative cooling for reducing condenser temperatures. Their analytical model was shown to be in agreement with the experimental data. One interesting test showed that, with the condenser heat rate held constant at 300 W (1024 Btu/h), the condensing temperature could be lowered from 44.6.degree. C. (112.3.degree. F.) with dry surfaces to 24.4.degree. C. (75.9.degree. F.) with wet surfaces. Also, the same data set showed that for a constant condensing temperature, the condenser heat could be increased from 300 W (1024 Btu/h) to 2280 W (7780 Btu/h) when applying the water spray. Although the increase in heat removal rate is partially offset by increased air pressure drop, the net effect is still very positive, with a stated possible decrease in air conditioner power consumption of 50%.